1. Field of the Invention
The present invention relates to a compound semiconductor device and a method for manufacturing the same and, more in particular, it relates to an electrode structure of a compound semiconductor, a semiconductor device such as an FET or HEMT, as well as a method of manufacturing the same.
2. Description of Related Art
In recent years, it has been highly demanded for miniaturization and reduction of electric power for terminals in a mobile communication system such as a portable telephone and, accordingly, a performance capable of attaining such miniaturization and reduction of electric power described above has also demanded for devices such as high frequency transistors used in the system. For example, with respect to power amplifiers for use in high frequency of 2 GHz band for digital cellulars having a leading position in current mobile communication, it has been demanded for devices capable that can operate on a single positive power source, can be driven at a lower voltage and at higher efficiency.
At present, a hetero junction field effect transistor (HFET) has been put to practical use as one of devices for high frequency power for use in high frequency power amplifiers in a microwave band and it is adapted to conduct current modulation utilizing hetero junction.
FIG. 5 shows an example for a constitution of HFET. In HFET, a first barrier layer 33 of AlGaAs mixed crystal, a channel layer 34 of InGsAs mixed crystal and a second barrier layer 35 of AlGaAs mixed crystal are laminated successively by way of a buffer layer 32 comprising a semi-insulative single crystals GaAs, and a gate electrode 40 is formed on the second barrier layer 35.
Each of the first and the second barrier layers 33, 35 has a carrier supply region containing n-type impurity 33a, 35a in high resistance region 33b, 35b respectively. When a voltage is applied to the gate electrode 40, a drain current flowing between the source electrode 38 and the drain electrode 39 is modulated in accordance with the change of the applied voltage. Further, in HFET, as shown in FIG. 5, the thickness of the second barrier layer 35 generally is reduced near the gate electrode 40 as a recessed structure, so that a region in which carriers are depleted or carriers are reduced compared with other channel regions is formed in the region of the channel layer just below.
In the HFET having such a structure, since carriers are accumulated in the channel layer 34 by applying a positive voltage to the gate electrode 40, it has a feature that the linearity of gate-source capacity Cgs and mutual conductance Gm to a gate voltage Vg is more excellent, in principle, compared with other devices, for example, junction FET (JFET) or Schottky junction FET (MES-FET: Metal Semiconductor FET). This can provide a significant advantage for improving the efficiency of power amplifiers.
Further, HFET of a structure as shown in FIG. 6 has also been proposed recently. In this structure, p-type impurities are selectively diffused to a portion just beneath a gate electrode 60, specifically, to a portion of a second barrier layer 55 corresponding to the recessed structure shown in FIG. 5, to form a p-type low resistance region 55C (impurity concentration: 1.times.10.sup.19 cm.sup.-3 or higher). The p-type low resistance region 55C is in contact with the gate electrode 60 and in the form of buried in the second barrier layer 55.
In such a structure, since a PN junction is used, a built-in volt is increased and a large positive voltage can be applied to the gate electrode 60, compared with a structure shown in FIG. 5 using a Schottky junction for the gate electrode 40. Accordingly, operation by a single positive power source can be facilitated while possessing excellent linearity of the mutual conductance Gm and the gate-source capacity Cgs of HFET as it is.
However, in the HFET structure in FIG. 6, the gate electrode 60 is in junction with the p-type low resistance region 55C formed in the second barrier layer 55. In a usual semiconductor of a large band gap (for example AlGaAs), it is difficult to obtain a satisfactory ohmic junction with the material for the gate electrode 60 used customarily (for example, a multi-layered structure: Ti/Pt/Au in view of the junction) compared with GaAs. As a result, the gate resistance is increased tending to cause degradation of high frequency characteristics.
In FIG. 6, elements 52, 53, 53a, 53b, 56, 58, and 59 correspond, respectively, to elements 32, 33, 33a, 33b, 36, 38, and 39 described above with reference to FIG. 5.